The invention relates to a differential amplifier comprising at least:
a first amplifier transistor whose base terminal is coupled to an emitter terminal of a first embitter-follower transistor, PA1 while base terminals of the emitter-follower transistors can be supplied with a differential voltage for controlling the differential amplifier. PA1 a third amplifier transistor whose base terminal is coupled to the base terminal of a first amplifier transistor, PA1 a fourth amplifier transistor whose base terminal is coupled to the base terminal of the second amplifier transistor, PA1 at least a second emitter impedance via which the emitter terminals of the third and fourth amplifier transistors are coupled to one another, and in that PA1 output currents of the differential amplifier can be derived from collector terminals of the third and of the fourth amplifier transistor.
a second amplifier transistor whose base terminal is coupled to an emitter terminal of a second emitter-follower transistor, PA2 a first emitter impedance across which the emitter terminals of the first and second amplifier transistors are coupled to each other,
In bipolar circuit technology it is known that an emitter negative feedback is used for linearizing differential amplifiers. Emitter terminals of bipolar transistors which form a differential amplifier are not coupled to each other direct but via at least one emitter impedance, which is preferably formed by an ohmic resistance. The larger the resistance value of this emitter impedance is, compared with the internal resistance of the transistors at their emitter terminals, the more linear the transfer characteristic becomes of a thus arranged differential amplifier. Since, however, with an increasing voltage swing of the transistors of the differential amplifier via their base terminals via a difference signal, the current in a respective one of the transistors of the differential amplifier decreases more and more and, consequently, its internal resistance at the emitter terminal increases, for these large voltage swings the linearization effect of the emitter impedance is curtailed accordingly.
FIG. 1 shows such an arrangement for an emitter-coupled differential amplifier which comprises a first amplifier transistor 1, a second amplifier transistor 2 and two emitter resistors 3, 4 which, connected in series, connect emitter terminals of the amplifier transistors 1, 2 to each other. A constant-current source 5, which is connected to ground 6 in the circuit diagram shown, is connected to the interconnected terminals of the emitter resistors 3 and 4 remote from the emitter terminals. Each of the emitter resistors 3 or 4 then forms an emitter impedance.
When the circuit arrangement shown in FIG. 1 is in operation, the constant-current source 5 produces a constant current I01. This constant current I02 is distributed over the collector emitter paths of the two amplifier transistors 1, 2 in accordance with the differential voltage DU applied between the base terminals of the amplifier transistors 1, 2, which differential voltage DU is used as an input voltage of the differential amplifier shown in FIG. 1. An increase of the current in the collector emitter path of one of the amplifier transistors, 1, 2 then directly leads to a reduction of the current in the collector emitter path of the other one of the amplifier transistors 1, 2. For the following exposition only the current deviations from their operating point which are caused by voltage swings are considered, i.e. deviations from half the value of the constant current I01. These deviations form an alternating current which is shown in FIG. 1 as a collector current IC or emitter current IE on the first amplifier transistor 1, while for simplicity of the exposition there may also be assumed that with sufficiently large current gain of the amplifier transistors 1, 2, these currents match in essence. The alternating current described, referenced IC or IE, thus flows from the collector terminal of the first amplifier transistor, via its collector emitter path, the two emitter resistors 3, 4 and the collector emitter path of the second amplifier transistor, but through the latter, in opposite direction compared to the first amplifier transistor 1, to the collector terminal of the second amplifier transistor 2.
FIG. 2 shows a basic circuit diagram of a transfer characteristic curve between the collector current IC of the first amplifier transistor 1 and the differential voltage UD at the base terminals of the amplifier transistors 1, 2, which characteristic curve can be achieved with such an emitter negative feedback differential amplifier arrangement. The emitter negative feedback by the emitter impedances 3, 4 provides a linearization of this characteristic curve in a range around its origin (UD=0, IC=0), but for larger voltage swings, i.e. larger values of UD, roundings are shown both in positive and in negative direction, i.e. non-linear influences in this characteristic curve.
A computation of the characteristic curve of FIG. 2 shows that the difference between the two base-emitter voltages of the amplifier transistors 1, 2 is included in the transfer function of the circuit arrangement shown in FIG. 1. This difference between the base-emitter voltages is non-linearly combined with the collector current IC and, as a result, provides a non-linear contribution to the characteristic curve shown in FIG. 2.
For a further linearization of the characteristic curve of a differential amplifier, a circuit arrangement is proposed as shown in FIG. 3. Elements included therein and corresponding to those of FIG. 1 are referenced by like characters. Also the circuit arrangement shown in FIG. 3 comprises a differential amplifier which comprises two amplifier transistors 1, 2 whose emitter terminals are interconnected via emitter resistors 3, 4, while a constant-current source connected to ground 6 is connected to a node between the emitter resistors 3, 4, which constant-current source produces a constant current I02 and is referenced 15. In FIG. 3 a first and a second emitter-follower transistor 11, 12 respectively, are added to this arrangement insofar as it corresponds to FIG. 1. The base terminal of the first amplifier transistor 1 is coupled to an emitter terminal of the first emitter-follower transistor 11, and the base terminal of the second amplifier transistor 2 is coupled to the emitter terminal of the second emitter-follower transistor 12. This coupling is established in FIG. 3 by a direct DC connection. Furthermore, the emitter terminal of the emitter-follower transistor 11 is connected to the collector terminal of the second amplifier transistor 2, and the emitter terminal of the second emitter-follower transistor 12 is connected to the collector terminal of the first amplifier transistor 1. As a result, there is a cross coupling or feedback in the way of a bistable multivibrator. From the emitter-follower transistors 11, 12 are now tapped from their collector terminals the collector currents used as output currents of the differential amplifier arrangement according to FIG. 3. The emitter currents IE and collector currents IC are again understood to be AC currents just like FIG. 1.
If in the circuit arrangement shown in FIG. 3 the transfer function is computed as a relation between the collector current IC and the differential voltage UD, which is now available via the base terminals of the emitter-follower transistors 11, 12, it turns out that the non-linear influences of the base-emitter voltages of the amplifier transistors 1, 2 compensate for the accordingly oppositely directed non-linear influences of base-emitter voltages of the emitter-follower transistors 11, 12, so that a considerably improved linearization of the characteristic curve between the collector current IC and the differential voltage UD is achieved. More particularly, the non-linear influences of the base-emitter voltages of the second emitter-follower transistor 12 cancel out those of the first amplifier transistor 1 and those of the first emitter-follower transistor 11 cancel out those of the second amplifier transistor 2. The collector current IC, and thus the emitter current IE, may then be well approximated according to Ohm's law from the differential voltage UD and the sum of the resistances of the emitter resistors 3, 4. However, the phase between these input and output magnitudes of the differential amplifier arrangement shown in FIG. 3, compared to the relation for the differential amplifier arrangement shown in FIG. 1 has turned through 180.degree., so that the result is a characteristic curve with a negative rise, as is shown in FIG. 4. Non-linear roundings of this characteristic curve when the value of the collector current IC approaches the constant current 102 are much smaller than respective distortions of the characteristic curve shown in FIG. 2.
However, it turns out that the circuit arrangement shown in FIG. 3 has disadvantages for certain operational situations. It can be noticed, for example, that when the arrangement shows a voltage swing with a differential voltage exceeding about 500 mV (for bipolar silicon transistors), the amplifier transistors 1, 2 become saturated. With an integration of such a differential amplifier arrangement and thus of the amplifier transistors 1, 2 on a semiconductor crystal effected in a general manner, high leakage currents may then evolve in the substrate of the semiconductor crystal, which lead to unwanted effects and are thus to be avoided at all costs. For this reason the dynamic range of such a differential amplifier arrangement is very limited. Furthermore, the structure of the differential amplifier arrangement shown in FIG. 3 by way of a bistable multivibrator, i.e. with a closed negative feedback loop, there is always the danger of instability, more particularly when relatively small resistances are selected for the emitter resistors 3, 4, which may be necessary, for example, to limit the voltages occurring across the emitter resistors 3, 4 in the case of a predefined collector current. In the borderline case the circuit arrangement shown in FIG. 3 then actually shows the behavior of a bistable multivibrator.
The fact that in the circuit arrangement shown in FIG. 3 the amplifier transistors 1, 2 become saturated relatively early furthermore provides that the phase of the collector current IC turns back to the phase position in accordance with the arrangement shown in FIGS. 1 and 2 when there are rather large voltage swings. If the differential amplifier arrangement shown in FIG. 3 is included in a closed loop control system, this phase shift occurring when there is saturation may lead to instabilities of the whole control system.